1. Field of the Invention
The present invention relates to a ball screw which is characterized by a boundary portion between a raceway and a return path.
2. Description of the Related Art
An example of a conventional ball screw is shown in FIG. 6 (which is a perspective view thereof) and FIG. 7 (a section view thereof) This is a ball screw of a tube type which uses a tube as a return path for balls; and the present ball screw is composed of a screw shaft 1, balls 3 and tubes 4. In FIG. 6, reference character 5 designates a tube holder for fixing the tubes 4 to a nut 2 and, in FIG. 7, illustration of this tube holder 5 is omitted.
In the outer peripheral surface of the screw shaft 1 and the inner peripheral surface of the nut 2, there are respectively formed spiral-shaped grooves 11, 21, while these spiral-shaped grooves 11, 21 cooperate together in forming a raceway for the balls 3. And, in case where the balls 3 roll on this raceway while they are loaded, the nut 2 is moved linearly with respect to the screw shaft 1. That is, the spiral-shaped grooves 11, 21 of the screw shaft 1 and nut 2 cooperate together in forming a ball rolling groove.
The tube 4 is formed substantially in a gate shape, while the two end portions of the tube 4 are respectively inserted into their associated penetration holes 22 formed in a cylindrical body which is used as the nut 2 and they are disposed diagonally opposite to each other across the screw shaft 1 so as to be able to connect together the start and end points of the raceway. This example has two ball circulation passages (a raceway+a return path) and, therefore, there are disposed two tubes 4.
FIG. 8 is a partial section view of the present ball screw, showing a boundary portion between the raceway and return path. In the nut 2, there is formed a penetration hole 22 facing from the outer peripheral surface thereof toward the ball rolling groove 21, while the end portion of the tube 4 is disposed in this penetration hole 22.
This penetration hole 22 has a stepped portion 22a for receiving the end face of the tube 4. That is, when the end portion of the tube 4 is mounted into the penetration hole 22, due to the stepped portion 22a of the penetration hole 22, the surface (the groove-side surface) 22b of the hole 22 formed on the ball rolling groove 21 side can be made to coincide with the inner surface 41 of the tube 4. The groove-side surface 22b corresponds to a rising surface where the end portion of the return path rises from the ball rolling groove 21 of the nut 2.
Also, in the end portion of the tube 4, there is formed a tongue 42 which, in a state where the tube 4 is mounted, is disposed opposed to the groove-side surface 22b of the penetration hole 22. This tongue 42 extends up to the vicinity of the bottom surface of the ball rolling groove 11 of the screw shaft 1 and provides a stop portion serving as the end portion of a raceway 30 which is composed of the ball rolling grooves 11, 21.
Therefore, a ball 3, which has arrived at the vicinity of the end point of the raceway 30, is butted against the leading end of the tongue 42 and is scooped up, is moved through between the groove-side surface 22b of the penetration hole 22 and tongue 42, and is guided into the tube 4. Also, the ball 3, which has moved through the tube 4, is then moved between the groove-side surface 22b of the penetration hole 22 and is guided into the raceway 30.
In the above-mentioned conventional ball screw of a tube type, there is a problem that the boundary portion between the rising surface of the return path (the groove-side surface 22b of the penetration hole 22) and the ball rolling groove 21 of the nut 2 hinders the smooth movement of the ball 3. To solve this problem, in JP-A-2001-14101, there is proposed a technique in which an angular boundary portion 25 is worked or shaven to turn it into a smooth shape (for example, such a surface shape as shown by reference character 27 in FIG. 8).
However, in the technique disclosed in the above-cited publication of patent application, there is room for improvement in the durability of the ball screw especially when it is rotated at a high speed.